Astronomers have long assumed that the gas surrounding the universe's first galaxies was still pristine — mostly hydrogen and helium, the only ingredients available in the newborn cosmos. A study published in Nature Astronomy by researchers at the University of Arizona suggests that picture is wrong.

Using the James Webb Space Telescope, the team found that galaxies were already producing and dispersing heavy elements such as oxygen, carbon and silicon just 500 million years after the Big Bang — when the universe was about 3% of its current age, during the Epoch of Reionization.

“We observed that heavy elements escaped from galaxies very, very early in cosmic time,” said Yongda Zhu, first author and a postdoctoral researcher at the U of A Department of Astronomy and Steward Observatory. “Not only were the galaxies producing these elements, but they were also dispersing them, possibly seeding other galaxies.”

Zhu's method was elegant: rather than hunting for emission, he used the galaxies themselves as background light sources. As their light travelled toward Earth it passed through surrounding gas, leaving faint absorption fingerprints. He manually searched publicly available JWST spectra from hundreds of galaxies and found three — at redshifts 7.2 to 9.3, their light more than 13 billion years old — with clear absorption features from O I, Si II, C II, Si IV and C IV. Nearly 30 hours of exposure were needed to detect signals that faint.

The absorption lines were blueshifted relative to each galaxy's systemic redshift, at relative velocities of 50–250 km/s. The mix of ionization states and overlapping velocity features points to disturbed, outflowing gas: galactic winds carrying metals out of the galaxies and into intergalactic space. Strikingly, the chemical fingerprints resemble those of galaxies billions of years more evolved — evidence that enrichment and outflow have been running since cosmic dawn.

The finding, part of a process known as baryon cycling, may also solve a long-standing puzzle: why astronomers have never confirmed an observation of Population III stars, the first generation forged from pristine gas. If galaxies were already enriching their surroundings 500 million years after the Big Bang, truly pristine gas may simply not have survived long enough to form stars we can still see.

“If you start out with pure vanilla ice cream but start mixing in sprinkles soon after, it won't be long until you can no longer find any pristine, plain, vanilla ice cream,” Zhu said.

Sources